1. Field of the Invention
The present invention relates generally to a zoom lens and, more particularly, to a zoom lens suitable for use with a lens shutter type-camera or the like.
2. Related Background Art
In recent years, a zoom lens has been dominated in the sector of photographing lenses of a lens shutter type camera.
Also, with a higher performance of the zoom lens, a variety of zoom types have been proposed. Further, because of an advancement of technology for a lens barrel in recent years, there have been proposed a variety of zoom types in which high zoom ratio is attained by use of a zoom lens constructed of three or more movable lens units, a so-called multi-unit zoom lens.
Given first is a generality about the multi-unit zoom lens including the three or more movable lens units.
In the multi-unit zoom lens, there increases freedom for selecting a zooming locus of each lens unit upon zooming from a wide angle end to a telephoto end, and, therefore, freedom in terms of correcting an aberration also increases. Further, because lens units changing magnification for zooming are increased, it is possible to attain a higher performance while increasing a zooming ratio. There has hitherto existed a problem in which a structure of a lens barrel becomes intricate with an increment in the number of movable elements. This problem is, however, overcome to some extent owing to the advancement of the technology for the lens barrel in recent years.
Hitherto, in the zoom lens with no restriction of a back focus, a negative lens unit has been disposed closest to an image side in order to reduce the total length of the lens system and to make the lens size small. The zooming is effectively performed by augmenting a variation in the back-focal distance upon zooming from the wide angle end to the telephoto end.
Further, a positive lens unit is disposed closest to the object side in the lens system, thereby attaining the reduction in the total length at the telephoto end.
From the above, a positive/positive/negative 3-unit zoom lens and a positive/negative/positive/negative 4-unit zoom lens may be specifically cited as a multi-unit zoom lens capable of increasing the zoom ratio and suitable for the down-sizing, and a variety of proposals pertaining thereto have been made.
The positive/positive/negative 3-unit zoom lens is constructed of, sequentially from the object side, a first lens unit having refractive power, a second lens unit having the positive refractive power and a third lens unit having negative refractive power. This zoom lens is constructed so as to, upon zooming from the wide angle end to the telephoto end, increase an air gap between the first lens unit and the second lens unit but to decrease an air gap between the second lens unit and the third lens unit (e.g., Japanese Patent Application Laid-Open No. 2-73211).
On the other hand, the positive/negative/positive/negative zoom lens is constructed of, sequentially from the object side, a first lens unit having the positive refractive power, a second lens unit having the negative refractive power, a third lens unit having the positive refractive power and a fourth lens unit having the negative refractive power. This zoom lens is constructed so as to, upon zooming from the wide angle end to the telephoto end, increase an air gap between the first lens unit and the second lens unit but to decrease both an air gap between the second lens unit and the third lens unit and an air gap between the third lens unit and the fourth lens unit (e.g., Japanese Patent Application Laid-Open No. 3-39920).
Also, in the zoom lens with no restriction in terms of the back-focal distance that is used for a lens shutter type camera, it is required for attaining the down-sizing of the camera body to reduce both a size of each lens and a thickness of the lens system (collapse lens thickness) when collapsed in the camera body. In this case, there are reduced both a length (total lens thickness) of the lens system along an optical axis extending from the closest-to-object surface to the closest-to-image surface and a thickness of each lens unit. These reductions are effective in the decrease in the collapse lens thickness and, in turn, the down-sizing of the camera body.
However, in the conventional multi-unit zoom lens such as the positive/positive/negative 3-unit zoom lens and the positive/negative/positive/negative 4-unit zoom lens, the negative lens element of the first lens unit is disposed closest to the object side in order to well correct a positive distortion at the wide angle end and obtain a sufficient back-focal distance. Then, if the first lens unit is constructed of two lens elements, a negative spherical aberration is insufficiently corrected. A well-corrected negative spherical aberration entails nothing but to increase the number of the constructive lens elements or to make the lens surface aspherical. As a result, there arises an inconvenience of being insufficient in terms of simplifying the configuration and reducing the costs.